1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to an improved thermosensitive recording material comprising a support, an undercoat layer formed on the support, and a thermosensitive coloring layer formed on the undercoat layer.
2. Discussion of Background
Recently, various information recording materials of a non-enviromental-pollution type, capable of nursing resources and economizing energy, have been developed and put to practical use for the purpose of dealing with a great variety of abundant information. In particular, thermosensitive recording materials have been widely employed in various fields, for instance, for use with terminal printers for computers and calculators, recorders for medical measurement instruments, low- and high-speed facsimile apparatus, automatic ticket vending apparatus, copying machines, and label printing machines for the POS system, because of the following advantages thereof:
(1) images can be readily recorded on a thermosensitive recording material by simply applying heat thereto without employing a complicated development process;
(2) a relatively simple and small-sized apparatus is available for preparing a thermosensitive recording material, handling of the recording material is easy, and the maintenance cost of the same is inexpensive; and
(3) in the case where paper, which is not costly as compared with other materials, is used as a support, a thermosensitive recording material with the plain paper-like touch is obtainable.
In general, the thermosensitive recording material is prepared by coating a liquid for forming a thermosensitive coloring layer, which contains a coloring component for color formation by application of heat, onto the surface of paper, synthetic paper or a plastic film, and then dried. Images are recorded on the thus prepared recording material by a thermal pen or thermal head. Such thermosensitive recording materials are disclosed, for instance, in Japanese Patent Publications Nos. 43-4160 and 45-14039. These conventional recording materials, however, sluggishly respond to thermal energy, so that images with high color density are not obtainable when high-speed recording is performed.
In order to overcome the above shortcomings, it have been proposed to incorporate the following compounds into the conventional thermosensitive recording materials: nitrogen-containing compounds such as acetamide, stearamide, m-nitroaniline and dinitrile phthalate (Japanese Laid-Open Patent Application No. 49-38424; acetoacetic anilide (Japanese Laid-Open Patent Application No. 52-106746); N,N-diphenylamine derivatives, benzamide derivatives and carbazole derivatives (Japanese Laid-Open Patent Application No. 53-11036); alkylated biphenyl and biphenyl alkane (Japanese Laid-Open Patent Application No. 53-39139); and p-hydroxy benzoate (Japanese Laid-Open Patent Application No. 56-144193). However, satisfactory recording materials cannot be obtained by incorporating any of the above compounds.
Japanese Laid-Open Patent Application No. 56-164890 discloses that the thermal sensitivity of a thermosensitive recording material can be improved by using an amorphous leuco dye, which is obtained by converting the corresponding crystalline leuco dye to the amorphous leuco dye, and has a lower melting point than that of the corresponding crystalline leuco dye. Such amorphous leuco dyes, however, are highly reactive because of their activated surfaces, so that a liquid for forming a thermosensitive coloring layer containing such an amorphous leuco dye causes a so-called liquid fogging, or the thermosensitive recording material containing the same causes the fogging of the background, thereby reducing the whiteness of the background.
In order to prevent the fogging of the background and to enhance the dynamic coloring sensitivity of the recording material, several methods described below have been proposed. For instance, a material having a thermal conductivity of 0.04 Kcal/mh.degree. C. or less is employed as a support as disclosed in Japanese Laid-Open Patent Application No. 55-164192; and a layer comprising fine hollow particles as the main component is formed on a support as disclosed in Japanese Laid-Open Patent Applications No. 59-5093 and 59-225987. However, the thermosensitive recording materials prepared in the above manners are not satisfactory because they have low flexibility, poor heat insulating properties, and low thermal head-matching ability.
Japanese Laid-Open Patent Application No. 62-5886 discloses a thermosensitive recording material comprising an intermediate layer which contains non-expandable fine hollow particles made of a thermoplastic resin, having a diameter of 5 .mu.m or less. The use of such hollow particles, however, is not a decisive manner for preparing a thermosensitive recording material having high sensitivity.